Documentation

If the client has indicated support for gzip or compress in its
Accept-Encoding header, and the Content-Type in the response is one of
the following types:

application/x-javascript

text/css

text/html

text/javascript

text/plain

text/xml

application/x-font-truetype

Then the given handler's output stream will be compressed,
Content-Encoding will be set in the output headers, and the
Content-Length will be cleared if it was set. (We can't process the stream
in O(1) space if the length is known beforehand.)

The wrapped handler will be run to completion, and then the Response
that's contained within the Snap monad state will be passed to
finishWith to prevent further processing.